1. Field of the Invention
The present invention relates to a method for producing alkali cellulose and a method for producing cellulose ether by using the alkali cellulose.
2. Description of the Related Art
A method for producing cellulose ether has been known in which a highly purified pulp is brought into contact with an alkaline solution to form alkali cellulose, and the alkali cellulose is subjected to etherification reaction by using an etherifying agent. The cellulose ether obtained at the end becomes soluble in water by adequately controlling the degree of substitution. However, a water-insoluble portion is still present therein to reduce the transmittance of the aqueous solution thereof or to reduce the commercial value as contamination.
The insoluble portion is caused by a low-substituted portion having insufficient amount of substituents to be dissolved in water. One of the causes is that an alkali distribution in the alkali cellulose is not uniform.
Activities of this alkali includes facilitating penetration of the etherifying agent by swelling cellulose to change the crystal structure in the pulp, catalyzing the etherification reaction with alkylene oxide, and becoming a reactant for alkyl halide. Accordingly, a portion of the pulp which has not been brought into contact with an aqueous alkali solution does not participate in the reaction and becomes an undissolved portion. Unevenness of alkali cellulose leads directly to the amount of undissolved portion.
A method for efficiently producing uniform alkali cellulose has been proposed in JP 2007-197682A. The method for producing alkali cellulose comprises the steps of continuously bringing a pulp into contact with an alkali metal hydroxide solution in a various type of contactor to obtain a contact product, and draining the contact product.
In the method, the alkali metal hydroxide solution recovered in the step of draining is usually reused or circulated for contact with a pulp.